1. Field of the Invention
This invention relates to a polyethylene composition having excellent physical and chemical properties, and excellent moldability. More particularly, it relates to a polyethylene composition which exhibits excellent processability as well as excellent properties such as high impact resistance, high environmental stress cracking resistance (hereinafter referred to as ESCR) and the like in the application of various molding techniques such as blow molding, extrusion molding, injection-blow molding and the like.
2. Description of the Prior Art
In molding by blow molding, extrusion molding, injection-blow molding or the like, polyethylene having relatively high molecular weight as well as relatively broad molecular weight distribution is suitable. Several processes have been proposed in the art for preparation of polyethylenes with broader molecular weight distributions.
One of the known processes comprises mixing a high molecular weight polyethylene and a low molecular weight polyethylene. Examples are described in Japanese Patent Publication No. 3215/1970, Japanese Patent Publication No. 22007/1970, Japanese Laid-open Patent Publication No. 100444/1979, Japanese Laid-open Patent Publication No. 100445/1979, Japanese Laid-open Patent Publication No. 161657/1979, Japanese Laid-open Patent Publication No. 60542/1980, Japanese Laid-open Patent Publication No. 60543/1980, Japanese Laid-open Patent Publication No. 57841/1981, and Japanese Laid-open Patent Publication No. 133136/1982. Another known process is a multistage polymerization process in which two or more stages of polymerization are involved. Examples of this latter mode of preparation are described in Great Britain Patent Publication Nos. 1,174,542 and 1,233,599, U.S. Pat. Nos. 4,113,440 and 4,098,974 and Japanese Laid-open Patent Publication No. 47079/1976. The polyethylenes prepared by these known processes have wide molecular weight distributions and produce moldings having a good ESCR.
The present inventors have found that although the prior polyethylenes have broad molecular distributions and improved ESCR, they exhibit a number of drawbacks with regard to practical characteristics required for both molding and molded shaped products. The drawbacks encountered in the prior polyethylenes are low impact strength, poor viscoelastic characteristics on melting during molding, and susceptibility to generation of thickness irregularity. In addition to these drawbacks, prior polyethylenes encounter failures in producing moldings with complicated shapes.